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Overview

Urbanization alters the natural soil profile, increases impervious surface areas and decreases vegetation cover. The losses of vegetation cover and the increase of impervious surfaces, such as roads, sidewalks, concrete buildings, and parking, result in increasing stormwater runoff, degrading water quality and impairing aquatic habitats(Asadian and Weiler, 2009). Consequently, these disruptions change the urban hydrological cycle and pose challenges to stormwater management in urban areas. With future population growth and climate change, there will be more intensified urbanization competing for land uses and more climate variability making extreme events such as drought and floods less predictable.

Urban trees play critical roles in regulating hydrological cycles and affecting surface water in the urban environment. They affect all hydrologic processes as well as the spatial and temporal redistribution of moisture (Xiao and McPherson, 2003). In particular, the interception of rainfall by urban trees is a major component of the surface water balance. The canopy rainfall interception changes the urban runoff process by slowing down the rainfall intensity and reducing the volume of water reaching the ground via water storage within the canopy surface (Sanders, 1986; Xiao and McPherson, 2003). The evaporation of intercepted water from tree canopies further reduces the amount of water entering the soil profile, and the extension of the tree root system increases macropores that facilitate soil infiltration. Therefore, urban trees decrease the peak flows and the total amount of urban runoff and reduce the potential damage caused by stormwater.